I tried (under Windows) to influence the language of the localized date string by setting LC_MESSAGES, LC_ALL, LC_TIME, … This works for changing the messages printed in the console, but don’t affect the date formatting when doing a checkout.

While searching a solution, I came up with a workaround: use fixed-length keyword substitution. That is, something like
$Date:: $, which will truncate the date to fill only the available space (and have the effect of truncating localized and potentially non-ASCII string at the end).
More info on this feature of SVN here, at the end of the page.

make[3]: Entering directory `/tmp/buildd/fonts-freefont-20120503+svn2520/sfd'
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/TrueType", line 20
SyntaxError: Non-ASCII character '\xe6' in file ../tools/generate/TrueType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
make[3]: Leaving directory `/tmp/buildd/fonts-freefont-20120503+svn2520/sfd'
make[3]: Entering directory `/tmp/buildd/fonts-freefont-20120503+svn2520/sfd'
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
File "../tools/generate/OpenType", line 20
SyntaxError: Non-ASCII character '\xe7' in file ../tools/generate/OpenType on line 20, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
make[3]: Leaving directory `/tmp/buildd/fonts-freefont-20120503+svn2520/sfd'
make[2]: Leaving directory `/tmp/buildd/fonts-freefont-20120503+svn2520'